Dynamics of charge separation in the excited-state chemistry of protochlorophyllide

Excited‐State Charge Separation in the Photochemical Mechanism of the Light‐Driven Enzyme Protochlorophyllide Oxidoreductase

“…This is accompanied by the appearance of broad excited-state absorption (ESA) bands on either side of the ground state bleach (GSB) band. The initial femtosecond-picosecondd ynamics,w hich represent the S2!S1 transition, vibrational relaxation, and formation of an ICT state,h ave been described previously [11][12][13][14][15] and are not the main focus of the current study.T he subsequent excited-state dynamics could be fitted to three sequentially evolving exponential functions with time constants of 390 ps, 3.2 ns,a nd 2.7 ms, confirming the model proposed previously. [11][12][13][14]15] The3 90 ps component is likely to represent the solvation of the ICT state.…”

“…The initial femtosecond-picosecondd ynamics,w hich represent the S2!S1 transition, vibrational relaxation, and formation of an ICT state,h ave been described previously [11][12][13][14][15] and are not the main focus of the current study.T he subsequent excited-state dynamics could be fitted to three sequentially evolving exponential functions with time constants of 390 ps, 3.2 ns,a nd 2.7 ms, confirming the model proposed previously. [11][12][13][14]15] The3 90 ps component is likely to represent the solvation of the ICT state. [11][12][13][14]15] This is then followed by decay of the S1/ICT excited state into along-lived triplet state in 3.2 ns,which is characterized by asmall apparent blue-shift in the GSB band of approximately 3nma nd the appearance of an ew ESA band at approximately 550 nm (Figure 2A).…”

“…[11][12][13][14]15] The3 90 ps component is likely to represent the solvation of the ICT state. [11][12][13][14]15] This is then followed by decay of the S1/ICT excited state into along-lived triplet state in 3.2 ns,which is characterized by asmall apparent blue-shift in the GSB band of approximately 3nma nd the appearance of an ew ESA band at approximately 550 nm (Figure 2A). [11][12][13]15] Thet riplet state then relaxes back to the ground state with alifetime of around 2.7 ms.…”

“…[3] An umber of time-resolved transient spectroscopy studies have identified different short-lived Pchlide* species,both in the isolated pigment [11][12][13][14][15] and in the ternary enzyme-substrate complex. [16][17][18][19][20] TheP chlide molecule is intrinsically very reactive with multi-exponential excited-state dynamics [11][12][13][14][15][16] and the formation of an umber of Pchlide excited-state species within afew nanoseconds,including an intramolecular charge-transfer (ICT) complex [11][12][13][14][15] and atriplet state. [13,15] A detailed understanding of the initial ultrafast steps in the ternary enzyme-substrate (POR-Pchlide-NADPH) complex that lead directly to the chemistry of Pchlide photoreduction has proved more challenging.…”

“…[16][17][18][19][20] TheP chlide molecule is intrinsically very reactive with multi-exponential excited-state dynamics [11][12][13][14][15][16] and the formation of an umber of Pchlide excited-state species within afew nanoseconds,including an intramolecular charge-transfer (ICT) complex [11][12][13][14][15] and atriplet state. [13,15] A detailed understanding of the initial ultrafast steps in the ternary enzyme-substrate (POR-Pchlide-NADPH) complex that lead directly to the chemistry of Pchlide photoreduction has proved more challenging. [16][17][18][19][20] Recent studies have shown [20] Moreover, conventional ultrafast pump-probe spectroscopy measurements are typically limited to af ew nanoseconds,w hich has prevented the identification of any long-lived excited-state intermediates prior to the hydride and proton transfer chemistry.T oo vercome this problem we have now used time-resolved UV/Vis and IR spectroscopy techniques that provide complete temporal resolution over the picosecondmicrosecond time range.Consequently,this has allowed us to identify all of the excited-state intermediates in the catalytic cycle of POR and to propose amechanism for harnessing the light energy to drive the subsequent hydride and proton transfer steps on the microsecond timescale.…”